mirror of https://github.com/saitohirga/WSJT-X.git
76 lines
3.4 KiB
Plaintext
76 lines
3.4 KiB
Plaintext
JTMS v3.0: Possible New Mode for Meteor Scatter
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1. Transmitting
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Type 1 messages are 72 user-information bits, source encoded as in
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JT65. Convolutional FEC (K=32, r=1/2) increases the number of bits to
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(72+31)*2 = 206. Nine bits are sent twice, extending the array to 215
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bits. These are interleaved by bit-reversal of index values. Then 43
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sync bits are inserted, spread evenly so as to fall at positions 1, 7,
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13, ... 253. Frame size is 258 bits: 215 information-carrying bits
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and 43 sync bits. Frame duration is 129 ms.
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[Optional: Type 2 messages convey 4 user information bits (report,
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R+report, RRR, 73) encoded with a (15,4,8) block code, plus a 12-bit
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CRC for each callsign encoded with the Golay (23,12) code. This
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makes for 15 + 2*23 = 61 information-carrying bits. One dummy bit is
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added, and the 62 bits are interspersed with 31 sync bits, making a
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frams of 93 bits and frame time 46.5 ms.]
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2. Modulation is BPSK at 2000 baud. For sample rate 48000 Hz, this
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means nsps = 48000/2000 = 24 samples per symbol. The baseband
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waveform is built as follows:
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a. Replicate each bit nsps times into array y(npts=30*48000),
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substituting -1 for 0. Repeat the whole message enough times
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to fill npts, then pad with zeros to length 2*npts.
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b. Compute real-to-complex FFT of y(2*nsym*nsps). Roll off the complex
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spectrum at f=1000 Hz. Translate the half-band 0-1000 upward to
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1500-2500 Hz, and insert conjugate values at 1500 down to 500 Hz.
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c. The inverse (complex-to-real) FFT yields the Tx audio waveform.
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3. Receiving
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a. Compute real-to-complex windowed FFTs, N=12288 (t=256 ms),
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stepped by 128 ms (say). Zap birdies, remove frequency
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components outside the range 300 - 2700 Hz, and convert to an
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analytic time-domain signal.
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b. Square the complex signal, cx2=cx*cx, and compute N=12288 FFT of
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cx2 (resolution = 3.9 Hz). Look for carrier at 3000 + 2*DF Hz
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+/- 2*Tol.
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c. If carrier is found, measure frequency f and phase phi. Multiply
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cx by exp(-twopi*i*f*t - phi) to recover the real baseband signal
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x() to within a sign ambiguity.
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d. Apply matched filter for the Tx pulse shape to x(). (This is just
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a rectangular BPF, 500 - 25-- Hz ?)
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e. Establish PSK symbol sync (offset i0, 0 to nsps-1 samples) by finding
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maximum of Sum(sum*sum) over groups of nsps consecutive samples.
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f. Read off the soft symbols, sym(1:512), and compute CCF with 3
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versions of the 43-bit sync vector (rotated by 0, 14, 29 out of
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its 43 positions) and three of the 31-bit sync vector (rotated by
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0, 10, 20 of 31).
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g. If the best CCF abs(peak) exceeds a specified threshold, the sign
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of peak resolves the sign ambiguity.
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h. For Type 1 messages: Gather the proper set of 215
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information-carrying soft symbols. Form averages using the 9
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extra symbols, reducing the number to 206, and remove
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interleaving to re-order the symbols. Then run the fano232
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decoder. If decoding fails, add soft symbols into an
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accumulation array and (if nsum is 2 or more) try decoding the
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average.
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i. For Type 2 messages: Gather the proper set of 62 soft symbols.
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Decode Nrpt using an exhaustive search over all possibilities.
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For the CRCs, also do exhaustive searches -- and make sure that
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the expected values are best (or fall in the top few, anyway).
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